BM25 is originally derived from the Binary Independence Model [1]}, which assumes that terms are mutually independent so that the relevance score of a query-document pair is a weighted combination of the matching score of each query term with respect to the document. This allows efficient document retrieval using data structures such as inverted index.
BM25 is also sometimes referred to as a family of BOW scoring functions with slightly different components and parameters. The function of Equation REF  is one of the most prominent instantiations. We refer the readers to  [2]} for a comprehensive review of sparse retrieval and indexing methods.
